The present invention relates to a coating system, in particular to a coating system for articles where plastic materials slide against flexible rubber materials.
The described coatings are particularly useful for coating containers for storage and administration of liquid protein solutions, such as insulin formulations.
Medical articles for dosing medical preparations, such as containers equipped with stoppers or plungers, require a smooth sliding movement of one component, e. g. a stopper, in contact with another component, e. g. a container wall, to provide reliable dosages with high precision. Often, surfaces of the components have been shown to develop an initial resistance to movement after being in contact for some time, and movement does not start until a certain applied force, hereafter called a static friction force, has been applied. The phenomenon causes a sudden, rapid relative movement of the two surfaces. Frequently, the movement stops and another resistance is built up. This kind of movement is generally known as the xe2x80x98slip-stickxe2x80x99 phenomenon and causes a problem in that it leads to irregular and imprecise dosages. The phenomenon is especially troublesome in dispensing devices where very small, dropwise dosages of protein solutions, e. g. insulin formulations, are required. If the xe2x80x98slip-stickxe2x80x99 phenomenon does not occur when the surfaces start to slide after the static friction force has been applied, the surfaces slide at a smoother rate by application of a so-called dynamic friction force.
Today large amounts of insulin are sold in dispensing devices. The insulin is filled in glass containers, which are equipped with rubber stoppers, and these containers are then loaded into dispensing devices. Usually, both the glass containers and the rubber stoppers are coated with silicone oil, poly(dimethyl siloxane) (PDMS), to reduce the friction between the container wall and the stopper. One common method to coat glass containers with silicone oil is to apply a PDMS-in-water emulsion and subsequently evaporate the water in an oven.
After the surfaces have been coated with for example a silicone oil and the plunger has been fitted to the container, the static friction force is relatively low. However, especially the static friction gradually increases with time to reach a plateau value, after which there is no further significant increase in the static friction. The time after which this plateau is reached depends on the specific system, often within a few hours. The reason for the increase in friction forces with the time elapsed after fitting the plunger in the container, is believed to be displacement of the silicone oils from the contacting surfaces by the squeezing force between the plunger and the container. The increase may also partly be due to a small absorption of the silicone oil by the rubber material.
It is an object of the present invention to provide a coating system providing a reduced friction at surfaces being in frictional engagement with each other which coating system is not displaced by the squeezing forces mentioned above. Furthermore, the present system is particular suitable for plastic/rubber combinations.
The object of the invention is obtained by a coating system for articles having at least a first component and a second component, said second component being in frictional engagement with said first component, wherein at least a part of the surface of the first or the second component is coated with a silicone oil based coating having a viscosity of at least 200,000 centistokes, i.e. a high viscosity coating.
Another object of the invention is a coating which is solid or almost solid comprising a silicone oil based block or graft copolymer, or segmented copolymer.
The coatings according to the present invention are especially suited to use for permanently coating internal surfaces of containers equipped with stoppers for storage and administration of liquid protein preparations. The coatings providing low friction between the two components, such as a container wall and a rubber stopper, result in high-precision dosing.
Another object of the invention is a process of producing a component of an article coated as described above, comprising
adding the coating material to the component material prior to molding and subsequently molding the component from the mixture, or
molding the component from the component material and subsequently applying the coating material to the at least one surface of the component.
A third object of the present invention is an article having at least a first component and a second component, said second component being in frictional engagement with said first component, wherein at least a part of the surface of the first or the second component is coated with a silicone oil based coating as defined above. Such an article is preferably a medical article, such as a container or an injection cylinder and a stopper.